The well-known conventional hydrostatic hydraulic transmission (H.H.T.) utilizes a fluid such as oil to transmit power from a pump or "input" portion of the transmission to the motor or "output" portion thereof. It typically consists of a positive displacement pump and a positive displacement motor, which can be of various standard types. When a power source such as an engine turns the input shaft, the pump pumps fluid to the motor which in turn rotates the load. Each pump and each motor has its "stator" half to transmit the holding torque to the stationary portion of the transmission during acceleration and deceleration. In some of these conventional transmissions, the "stator" portion is allowed to rotate when not required to provide a "holding" torque.
It is universally recognized that the pumping and motoring of the fluid give rise to internal friction, and represent substantial inefficiencies.
It is also recognized that, although large speed ratios are attained in convention H.H.T.s, high torque ratios over a wide speed range are not feasible. The usual combinations are:
(a) low speed output with high torque ratio; PA1 (b) medium speed output with medium torque ratio; PA1 (c) high speed output with low torque ratio.
In a conventional H.H.T. having a high torque ratio and a wide speed range, the output motor is sized to produce the high starting torque, and the input pump is then sized to match the volumetric capacity required by the output motor. At low speed and high torque, the pump is larger than required, and at high speed and low torque, the motor is larger than required. This results in increased initial costs and weight, as well as inefficiencies in pumping and motoring the fluid.
During deceleration of the load, the input pump reverses its role and becomes a motor, and the output motor also reverses and becomes a pump. For ease of description, the input component will be referred to hereinafter as a pump, and the output component will be referred to hereinafter as a motor.